diff --git a/forum/migrations/0004_auto_20170530_2058.py b/forum/migrations/0004_auto_20170531_1949.py
similarity index 60%
rename from forum/migrations/0004_auto_20170530_2058.py
rename to forum/migrations/0004_auto_20170531_1949.py
index d236d5ac..0ae64560 100644
--- a/forum/migrations/0004_auto_20170530_2058.py
+++ b/forum/migrations/0004_auto_20170531_1949.py
@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 from django.db import migrations, models
+import django.db.models.deletion
 
 
 class Migration(migrations.Migration):
@@ -22,41 +23,41 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='forum',
             name='_last_message',
-            field=models.ForeignKey(related_name='forums_where_its_last', null=True, to='forum.ForumMessage', verbose_name='the last message'),
+            field=models.ForeignKey(verbose_name='the last message', to='forum.ForumMessage', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='forums_where_its_last'),
         ),
         migrations.AddField(
             model_name='forum',
             name='_topic_number',
-            field=models.IntegerField(verbose_name='number of topics', default=0),
+            field=models.IntegerField(default=0, verbose_name='number of topics'),
         ),
         migrations.AddField(
             model_name='forummessage',
             name='_deleted',
-            field=models.BooleanField(verbose_name='is deleted', default=False),
+            field=models.BooleanField(default=False, verbose_name='is deleted'),
         ),
         migrations.AddField(
             model_name='forumtopic',
             name='_last_message',
-            field=models.ForeignKey(related_name='+', null=True, to='forum.ForumMessage', verbose_name='the last message'),
+            field=models.ForeignKey(verbose_name='the last message', to='forum.ForumMessage', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+'),
         ),
         migrations.AddField(
             model_name='forumtopic',
             name='_message_number',
-            field=models.IntegerField(verbose_name='number of messages', default=0),
+            field=models.IntegerField(default=0, verbose_name='number of messages'),
         ),
         migrations.AddField(
             model_name='forumtopic',
             name='_title',
-            field=models.CharField(max_length=64, verbose_name='title', blank=True),
+            field=models.CharField(max_length=64, blank=True, verbose_name='title'),
         ),
         migrations.AlterField(
             model_name='forum',
             name='description',
-            field=models.CharField(max_length=512, verbose_name='description', default=''),
+            field=models.CharField(max_length=512, default='', verbose_name='description'),
         ),
         migrations.AlterField(
             model_name='forum',
             name='id',
-            field=models.AutoField(serialize=False, db_index=True, primary_key=True),
+            field=models.AutoField(primary_key=True, serialize=False, db_index=True),
         ),
     ]
diff --git a/forum/models.py b/forum/models.py
index 9f0f76e5..b7f0c8d8 100644
--- a/forum/models.py
+++ b/forum/models.py
@@ -58,7 +58,8 @@ class Forum(models.Model):
     view_groups = models.ManyToManyField(Group, related_name="viewable_forums", blank=True,
             default=[settings.SITH_GROUP_PUBLIC_ID])
     number = models.IntegerField(_("number to choose a specific forum ordering"), default=1)
-    _last_message = models.ForeignKey('ForumMessage', related_name="forums_where_its_last", verbose_name=_("the last message"), null=True)
+    _last_message = models.ForeignKey('ForumMessage', related_name="forums_where_its_last",
+            verbose_name=_("the last message"), null=True, on_delete=models.SET_NULL)
     _topic_number = models.IntegerField(_("number of topics"), default=0)
 
     class Meta:
@@ -181,7 +182,8 @@ class ForumTopic(models.Model):
     forum = models.ForeignKey(Forum, related_name='topics')
     author = models.ForeignKey(User, related_name='forum_topics')
     description = models.CharField(_('description'), max_length=256, default="")
-    _last_message = models.ForeignKey('ForumMessage', related_name="+", verbose_name=_("the last message"), null=True)
+    _last_message = models.ForeignKey('ForumMessage', related_name="+", verbose_name=_("the last message"),
+            null=True, on_delete=models.SET_NULL)
     _title = models.CharField(_('title'), max_length=64, blank=True)
     _message_number = models.IntegerField(_("number of messages"), default=0)